Method of monitoring the quality of filler gases, in particular sulphur hexafluoride, in gas-filled installations

ABSTRACT

A method of monitoring the quality of filler gases, in particular of sulfur hexafluoride (SF 6 ), in a gas-filled installation, in particular in gas-insulated switch units, such as high and medium voltage switches, a solution is provided which enables a continuous monitoring of the quality of filler gases in the gas-filled installation, which simultaneously enables to possibly determine the location of a fault and a type of the fault in such an installation. This is achieved by virtue of the fact that the filler gas of the installation, at least for the time of the analysis, is in a condition of a constant gas exchange with an ion-mobility spectrometer, in which the filler gas is ionized, and the ions are finally analyzed in the drift channel of the ion-mobility spectrometer.

The present invention relates to a method of monitoring quality offiller gases, in particular of a sulfur hexafluoride (SF₆), in agas-filled installation, in particular in gas-insulated switch units,such as high and medium voltage switches.

In gas-insulated high and medium voltage installations or containers,the filler gases are put in as insulation gases. At that, preferablysulfur hexafluoride (SF₆) is used because SF₆ combines a number ofimportant characteristics which favors its use such as high insulationproperty, distinguished arc quenching characteristics, non-toxicity,high thermal and chemical stability, a small dielectric loss factor,high gas density, and favorable heat transfer characteristics. ThoughSF₆ has, under normal conditions, a constant chemical bonding, highthermal loads and electrical discharges destroy the SF₆ molecules.Gas-insulated switch units, therefore, provide for a qualitativeretaining of insulation. However, during an operation, one cannot insurethat this gas would not decompose if suitable counter-measures are nottaken. This decomposition can be caused, e.g., by overheating at aconnection location. Partial discharges on defected screen electrodes ordirect gas breakdowns should be avoided. The decomposition productsproduced in this way are partially very high toxic and very corrosive.In addition, they adversely affect the electrical strength of the SF₆insulating gas in switch apparatuses and installations which, togetherwith increasing dissipation of the installation-filling gas determinesthe operational reliability of distribution of electrical energy.

These exemplary and known negative effects on the insulating property ofthe SF₆ gas require that very high standards be observed during themanufacture of the SF₆ installations and a constant monitoring of theinsulating gas. It is known that despite high manufacturing quality andthe undertaken measures such as fitting in of molecular sieves orabsorption fitters in the installation in order to tie up water orgaseous decomposition products, long-life characteristics of theinstallation can deteriorate. Thus, e.g., partial discharges ingas-insulated switches can be measured directly. Because the formationof gaseous decomposition products worsens the electrical strength of aSF₆ installation, the quantative data about the electrical strength ofSF₆ gas with decomposition products is of a big interest for powersuppliers as operators of such an installation. The same is true fornumerical values of reduction of the withstand voltage of the pininsulators under the influence of the humidity and the decompositionproducts. These data are indispensable for a gas diagnostics, e.g., fordetermining the time point of a gas exchange or of an event controlledrevision as a long-range objective. A further planned use of the gasdiagnostics is determining the fault location and the type of the fault.The fault location is found based on concentration differences inseparate selected regions of the installation. The type of the fault canbe determined from separate components of the decomposition products.Quantitative data related to the electrical strength of SF₆ gasincluding impurities of its decomposition products are obtained fromexperiments with a direct current corona discharge in a scientificapparatuses and off-line sampling and analysis in electrical switchunits. To this end, test tubes for the web chemical method and theFourier-transform-infrared spectroscopy as well as mass spectrometry areused. Conclusions based on acoustic methods with regard to the qualityof the isolation gas are labor-consuming and instrumentation-consuming,are not completely precise and, at that, are relatively expensive. Allof the above-mentioned methods are gas analytic laboratory methods,which are highly delicate and have a good selectivity, are off-linemethods and, therefore, can indicate the operational condition of aninstallation only with a time delay. In addition, they are bothlabor-intensive and costly.

The object of the invention is, therefore, monitoring of the quality offiller gases in a gas-filled installation which would enable todetermine the fault location and the type of a fault in such aninstallation.

According to the invention, this object is achieved by a continuousexchange at least at a time of analyses, of the filler gas of theinstallation with an ion-mobility spectrometer in which the filler gasis ionized and finally analyzed in the drift channel of the ion-mobilityspectrometer. The use of the ion-mobility spectrometer for otherpurposes has been known for a long time. This apparatuses for some yearshave been designed and manufactured for use for military and civilpurposes. Nowdays, the ion-mobility spectrometer is primarily used as awarning apparatus for chemical warfare products. Different scientificworks describe the mobility of different negative and positive ions,which are primarily formed in SF₆, at different temperatures andpressures. However, here, the pressures are not comparable withoperational pressures in high-voltage switches (P. L. Peterson,Mobilities of Negative ions in SF₆, J. Chem. Phys. 53 (1970) 694-704),and the temperatures are much higher than in the SF₆ -filledinstallations. (S. N. Lin, G. W. Griffin, E. C. Horning, W. E.Wentworth, Dependence of Polpyatomic Ion Mobility on Tonic Size, J.Chem. Phys. 12 (1974), 94494-44999), or the electrical strength does notcorrespond to that in the ion-mobility spectrometers (J. deUrquijo-Carmona, C. Cisneros, J. Alvarez, Measurement of Tonization,Positive Ion Mobility and Longitudinal Diffusion Coefficients in SF₆ atHigh E/N, J. Phys. D:Appl. Phys. 18 (1985) 92017-2022).

The method according to the present invention makes possible acontinuous and reliable monitoring of a quality condition of anindustrial SF₆ -filled installation. Ion-mobility spectrometry is amethod in which the ions are supplied, under ambient pressure, by directionization or ion-molecular reactions and are then analyzed in the driftchannel. The time which the ions require to produce a certain track inthe drift channel, under the influence of an electrical field, isprimarily a function of the mass and the charge of the ions. Inaddition, a certain role is played by the charge distribution in themolecule and by polarization.

An ion-mobility spectrometer can be so adapted, continuously ortemporarily, to a gas-filled installation, which it bypasses that thespectra in the millisecond range of the examined mixtures can be takenand then evaluated in a suitable manner. Here, the observed differencebetween the spectrum of a pure SF₆, which practically corresponds in thefilled installation to the spectrum contained in the middle, and theactual spectrum is used. In the spectrum, separate ions can be selectedas conductive components. When in the course of an operation of agas-filled installation, a change in the quality of the filler gasoccurs, then the ion-mobility spectrum would change. Additional peakscan occur, dependent of the nature of the filler gas, its quality, itsfilling pressure, its water content, its oxygen content, and otherparameters, or shifting of the peak position can take place. The courseof the changes can be monitored and evaluated with computer assistance.Also, the quality of a purified SF₆ can be checked and monitored duringrefilling of the SF₆ -filled installations.

Advantageously, the detected filler gas compound can be compared with areference gas compound, and when a predetermined deviation from thereference gas compound is exceeded, a fault-signal is generated in orderto signalize need for inspection and a danger of failure of theinstallation. As a reference gas naturally pure SF₆ is used.

In the ionization chamber of an ion mobility spectrometer, the primaryionization of gas molecules of the filler gas is effected withβ-radiation, UV-radiation or with partial electrical discharge, with thelatter being particularly advantageous.

According to a further advantageous embodiment, it is contemplated, whenthe ionization is effected by the electrical partial discharges, thatthe number and the duration of the partial discharges are so controlledwith a series of electrical pulses that the initial pulses for an ioncluster directly coincides with the pulses of the partial discharges. Inaddition, the number of discharge carriers can be controlled, wherebypreferable or intended ionization results are achieved, when aradioactive radiation source is used.

It proved to be particularly advantageous when a constant electricalfield with a field strength between several 10 and several 100 up toseveral 1000 V/cm is established in the drift space of an ion-mobilityspectrometer.

In order to insure a continuous monitoring, the ion-mobilityspectrometer can be embedded in the gas-filled region of theinstallation or be in condition of a continuous gas exchange with thefiller gas via a gas conduct (by gas flow and/or gas diffusion). Suchgas exchange should be present or established at off-line and/or on-sideoperation, e.g., when periodical monitoring takes place.

It proved to be particularly advantageous for effecting theabove-described process when an ion-mobility spectrometer has arod-shaped ionization source located in the ionization chamber and aplate-shaped collector electrode at the end of the drift chamber(point-plate arrangement). In this way, the ions can be provided in thecarrier gas SF₆ directly in the reaction chamber by a partial discharge,whereby a constant gas exchange between the filler gas of theinstallation and the ion-mobility spectrometer can be so establishedthat a continuous measurement, at least for the time of analysis,becomes possible.

It proved to be particularly advantageous when the drift chamber isformed as a cylindrical tube, with the drift chamber beingadvantageously formed of a plurality of conductive electrode ringsseparated by insulated intermediate rings. Such an electrode arrangementis basically disclosed in DE 41 30 810 Cl.

Now, the invention will be explained in detail below with reference tothe drawings. The drawings show:

FIG. 1 a schematic diagram of a gas-insulated high-voltage switchequipped with an ion-mobility spectrometer; and

FIG. 2 a diagram of comparison of ion-mobility spectra for negative ionsin sulfur hexafluoride for pure SF₆ for a sample from an actual switchunit and for a laboratory-aged gas.

FIG. 1 shows a simplified view of a gas-insulated high-voltage switchdesignated as GIS. This high-voltage switch is connected, by a suitableconnection 2, with an ion-mobility spectrometer IMS for enabling gasexchange therebetween. The ion-mobility spectrometer JMS has arod-shaped ionization source 3 which is arranged in an ionizationchamber with is bounded at the other end by a switch grid 4 for formingion clusters. A further grid, not shown, can be arranged between theionization source 3 and the switch grid 4. The switch grid 4 is adjoinedby a drift tube 5 having a tubular shape and formed of a plurality ofconductive electrode rings separated with insulated intermediate rings,respectively. At the other end of the drift tube 5, a Faraday plate 6,which functions as a collector electrode, is arranged.

In order to establish a uniform drift field of a suitable strength alongthe axis of the conductive electrode rings, the conductive electroderings can be connected to a suitable potential. This potential isgenerated by a high-voltage source and, as a rule, is adjusted by asuitable resistor chain or resistor network. The strength of theelectrical field is in order of several 100 V/cm along the drift path.

The strength of the electrical field in ionization chamber need notnecessarily correspond to that in the drift chamber. An aperture grillis (not shown in the drawings) is provided in front of the Faraday plate6 (the collector electrode).

Usually, a carrier gas with sample molecules, here decompositionproducts in the carrier gas SF₆ is supplied into the ionization chamberof the ion-mobility spectrometer IMS. This can be effected through thesuitable connection 2 by a continuous gas exchange with the SF₆ -filledinstallation GIS by way of gas transfer (gas flow or gas diffusion).

In the ionization chamber of the ion-mobility spectrometer IMS, carriergas and analysed gas ions (decomposition products) are formed by using aradioactive radiation source, a UV-light source, or corona discharge, orparticularly preferable a partial discharge. In the reaction region, asa rule, there is provided such electrical potential gradient, that thecharged mixture of different ions is displaced toward the injectiongrill. For forming ion clusters, a switch grid 4 is located at theoutlet of the ionization chamber. It opens periodically, e.g., every 50ms for several 10 to 100 μs; normally, however, the passage to the driftcell is closed. From the adjacent outer electrical field, during thetime the grid is open, a certain amount of ion mixture flows into thedrift chamber 5. The outer electrical drift field, which is maintainedsubstantially constant and has, in an ideal case, a linear potentialgradient, causes a rapid drift of the ions, which may have differentmass and structure and which move through an aperture grill, not shown,at the end of the drift chamber toward the collector electrode (Faradayplate 6). In a favorable case, completely separated partial ion clustersare located on the Faraday plate 6 where an ion-mobility spectrum can beobtained with a suitable amplifier and a display means. Here, thedependence of the time of arrival of the ions on their mobility is used.Thus, lighter ions reach the Faraday plate 6 earlier than heavier ions.

The possibility of on-line monitoring of the quality of SF₆ gases in thegas-insulated high-voltage switch GIS is based on the fact that ions canbe made directly available in carrier gas SF₆ in the reaction chamber bypartial discharge, advantageously with a point-plate device shown inFIG. 1, and on the fact that a continuous gas exchange between thefiller gas of the high-voltage switch GIS and the ion-mobilityspectrometer IMS can be so established that a continuous measurementbecomes possible. Here, the difference between the spectrum in a pureSF₆, which is maintained in the middle of the filled installation, andthe actual spectrum is utilized. At that, single ions can be screened inthe spectrum as conductive components. When during the operation of thegas-filled installation, the quality of the filler gas changes, theion-mobility spectrum also changes. Additional peaks can occur dependenton the nature of the filler gas, its quality, its pressure, its watercontent, its oxygen content, and other parameters. Alternatively, thepeak position can change. The change course can be monitored anddetermined with computer assistance. When the set threshold values for acorresponding installation are exceeded, a need for inspection issignalized, and the danger of a failure of the installation and/or theswitch GIS is displayed and, if necessary, an alarm is triggered.

FIG. 2 shows typical spectra, which occur in practice, namely, a curvefor pure SF₆ (purity 99, 98%), for a sample from a real gas-insulatedhigh-voltage switch GIS, and for a sample of laboratory-aged and,therefore, very damages SF₆. The differences are clearly recognizable,in particular, in the shift of the peak maximums, which makes possibleto evaluate the quality of the SF₆ gases.

Naturally, the method according to the invention can be used not onlyfor monitoring the quality of SF₆ but also for other gases.

We claim:
 1. A method of monitoring the quality of filler gases, inparticular sulfur hexafluoride (SF₆) in gas-filled installations, inparticular in gas-insulated switch units such as high and medium voltageswitches, characterized in that the filler gas of the installation, atleast for the time of analysis, is in a condition of a continuous gasexchange with an ion-mobility spectrometer in which the filler gas isionized and, finally, is analyzed in a drift channel of the ion-mobilityspectrometer.
 2. A method according to claim 1, characterized in thatthe already detected filler gas composition is compared with a referencegas composition and when a set deviation from the reference gascomposition is exceeded, an error signal is generated.
 3. A method,according to claim 1, characterized in that a primary ionization of gasmolecules of the filler gas is effected in an ionization chamber of theion-mobility spectrometer by β-rays, UV-rays or by an electrical partialdischarge.
 4. A method according to claim 3, characterized in thatduring the ionization by an electrical partial discharge by a series ofelectric pulses, the number and the duration of the partial discharge isso controlled that the initial pulse for an ion cluster directlycoincides with the pulse of the partial discharge.
 5. A method accordingto claim 1, characterized in that a constant electrical field having afield strength between several 10 and several 100 to 1000 V/cm isestablished in a drift chamber of the ion-mobility spectrometer.
 6. Amethod according to claim 1, characterized in that the ion-mobilityspectrometer is embedded in a gas-filled region of the installation oris in condition of a continuous exchange with the filler gas via a gasconduit.
 7. An ion-mobility spectrometer for effecting the methodaccording to claim 1, characterized in that it comprises a rod-shapedionization source (3) in the ionization chamber and a plate-shapedcollector electrode at the end of the drift chamber(s) (point-platedevice).
 8. AN ion-mobility spectrometer according to claim 7,characterized in that the drift chamber (5) is formed as a cylindricaltube.
 9. An ion-mobility spectrometer according to claim 8,characterized in that the drift chamber (5) is formed of a plurality ofconductive electrode rings separated with insulation intermediate rings,respectively.